The art of printing images with micro-fluid technology is relatively well-known. In thermal inkjet printing technology, thermal inkjet printers apply ink to a print medium by ejecting small droplets of ink from an array of nozzles located in a printhead. An array of thin-film resistors on an integrated circuit on the printhead selectively generates heat as current is passed through the resistors. The heat causes ink contained within an ink reservoir adjacent to the resistors to boil and be ejected from the array of nozzles associated with the resistor array. A printer controller determines which resistors will be “fired” and the proper firing sequence thus controlling the ejection of ink through the printhead so that the desired pattern of dots is printed on the medium to form an image.
For the ink supply, ink in thermal inkjet printers using an on-carrier ink supply system may be contained in printhead cartridges which include integrated ink reservoirs. The printhead cartridges are mounted on the carriage which moves the printhead cartridges across the print medium. The ink reservoirs often contain less ink than the printhead is capable of ejecting over its life. Printhead cartridges, together with the printhead, are replaced when the ink is depleted. However, the useful lifetime of a printhead can be extended significantly if the integrated ink reservoir can be refilled. Several methods now exist for supplying additional ink to the printhead after the initial supply in the integrated reservoir has been depleted. Most of these methods involve continuous or intermittent siphoning or pumping of ink from a remote ink source to the print cartridge. The remote ink source is typically housed in a replacement ink tank which is “off-carrier,” meaning it is not mounted on the carriage which moves the printhead cartridge across the print medium. In an off-carrier ink supply system, the ink usually travels from the remote ink tank to the printhead through a flexible conduit.
In an off-carrier ink supply system, air inadvertently enters the printhead reservoir with the ink. Air bubbles containing liquid vapor are formed spontaneously through cavitation or nucleation during the printing operation. Air is also entrained during ejection of ink through the nozzles. Air along the ink path and those trapped in the pre-ejection chamber or via are among the major problems in inkjet printing. Air bubbles grow by rectified diffusion and eventually interfere with the flow of fluid to the nozzles, leading to a breakdown of the jetting process.
For the printhead to operate properly, air must be periodically removed. Among the known methods of removing air is to attach a vacuum source to the printhead to suck air from the fluid supply line through a vent. The vent allows air to pass through but not liquids. In removing air in the pre-ejection chamber or via, a suction cap and pump are engaged to periodically remove air from the printhead through the nozzles. This method is known as priming. During priming, air is sucked through the nozzle. When removing the air during priming, a certain amount of ink is inadvertently sucked in the process. During every cap suction process significant quantities of ink is wasted. This results in poor ink use efficiency. As the length of nozzle arrays becomes longer, the pre-ejection chamber or via becomes longer and shallower and the volume of entrained air increases which requires frequent priming or a much bigger suction cap and pump, otherwise, entrained air accumulates and could be trapped in the pre-ejection chamber and could choke off the ink flow to the nozzles of the printhead. Frequent priming or a much bigger suction cap and pump result in increased volume of waste ink.
Accordingly, a need exists in the art for a microfluid ejection system which effectively removes air from the printhead and also improves ink use efficiency.